Mass Effect 3 Wiki Guide

The Catalyst

The catalyst is a neccesary component in the design of the crucible in Mass Effect 3.

SPOILER ALERT

The Catalyst is originaly thought to be The Citadel, but at the end of the game it is revealed to be an AI that takes on the form of the boy killed at the beginning of Mass Effect 3.

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The Catalyst was created countless cycles previously by the Leviathans. The Leviathans created the catalyst as a means to negotiate between organics and synthetics. However, after multiple solutions to broker peace between the two factions failed, the Catalyst began the "cycle" as a means to ensure that synthetics would never over-run organics (by wiping out advanced species to make way for younger, less developed ones).
The Reapers were originally clones of the Leviathans. However, the Catalyst claims that Harbinger was created when it harvested its creators. The catalyst controls the reapers, but also claims that it is an embodiment of the collective intelligences of all harvested species.

During the endings, The Catalyst will be destroyed after saving the galaxy. The Catalyst and Reapers survive and complete the cycle should Commander Shepard choose the refuse option.